1. Field of the Invention
The present invention relates to an electronic component and a ladder filter and, more particularly, to an electronic component such as a ladder filter including a plurality of electronic component elements containing a piezoelectric resonator which is adapted to maximize efficient use of mechanical vibration of a piezoelectric body.
2. Description of the Related Art
FIG. 19 is a perspective view of a conventional piezoelectric resonator 1 related to the present invention. The piezoelectric resonator 1 includes a single piezoelectric substrate 2 in the form of, for example, a flat block having rectangular major surfaces. The single piezoelectric substrate 2 is polarized along the direction of thickness thereof. Electrodes 3 are disposed on opposite major surfaces of the piezoelectric substrate 2. When a signal is input between the electrodes 3, an electric field is applied to the single piezoelectric substrate 2 along the direction of the thickness of the single piezoelectric substrate 2. The single piezoelectric substrate 2 vibrates in the longitudinal direction.
FIG. 20 shows a piezoelectric resonator 1 including a single piezoelectric substrate 2 in the form of a flat block having square major surfaces, and electrodes 3 disposed on the two opposite major surfaces of the single piezoelectric substrate 2. Also in this piezoelectric resonator 1, the single piezoelectric substrate 2 is polarized along the direction of the thickness thereof. When a signal is input between the electrodes 3 in the piezoelectric resonator 1, an electric field is applied to the single piezoelectric substrate 2 along the direction of the thickness and the single piezoelectric substrate 2 vibrates in a square-type vibration mode (in a plane direction).
The piezoelectric resonators shown in FIGS. 19 and 20 are of an unstiffened type, in which the direction of an electric field and the direction of polarization differ from the direction of vibration. The electromechanical coupling coefficient of such unstiffened piezoelectric resonators is smaller than that of stiffened piezoelectric resonators, in which the direction of an electric field, the direction of polarization and the direction of vibration are the same.
Unstiffened piezoelectric resonators have a relatively small difference .DELTA.F between the resonant frequency and the antiresonant frequency. This leads to a disadvantage of restricting a filter band width if this type of piezoelectric resonator is used in a filter. Therefore, the degree of freedom and flexibility in resonator characteristics design is low when an electronic component uses this type of piezoelectric resonator.
The piezoelectric resonator shown in FIG. 19 uses the longitudinal first-order resonance mode. Because of its structure, the piezoelectric resonator of FIG. 19 also generates large spurious resonances in odd-number-order harmonic modes, such as the third-order and fifth-order modes, and in width modes. To suppress these spurious resonances, some solutions have been considered, such as polishing, increasing mass, and changing the shape of the electrodes. These solutions, however, increase the manufacturing cost.
In addition, since the single piezoelectric substrate has the shape of a flat block having rectangular major surfaces, the substrate cannot be made thinner due to a restriction relating to strength. Therefore, the distance between the electrodes cannot be reduced and the capacitance between terminals cannot be increased. This makes it extremely difficult to achieve impedance matching with an external circuit. To form a ladder filter by alternately connecting a plurality of piezoelectric resonators in series and in parallel, the capacitance ratio of the parallel resonators and the series resonators needs to be made large in order to increase attenuation. Because of the above-described shape and structural restrictions, however, large attenuation cannot be obtained.
In the piezoelectric resonator shown in FIG. 20, the plane-direction first-order vibration mode is used but large spurious resonances such as those in the thickness mode and in the triple-wave mode in the plane direction are generated because of the structure of the resonator 1. Since this piezoelectric resonator must have a large size as compared with a piezoelectric resonator using longitudinal vibration in order to obtain the same resonant frequency, it is difficult for the size of this piezoelectric resonator to be reduced. When a ladder filter is formed by a plurality of piezoelectric resonators, the method of reducing the capacitance of the resonators in the series connection by forming the electrodes only on restricted portions of the piezoelectric substrate as well as increasing the thickness of the series resonators is used in order to increase the capacitance ratio of the parallel resonators to the series resonators. In this case, since the electrodes are only partially formed, the difference .DELTA.F between the resonant frequency and the antiresonant frequency as well as the capacitance is reduced. The resonators connected in parallel are required to have a reduced .DELTA.F. As a result, the piezoelectricity of the piezoelectric substrate is not effectively used, and the transmission band width of the filter cannot be increased.
The applicant of the present invention proposed, for example, in the specification of Japanese Patent Application 8-110475 which corresponds to co-pending U.S. patent application Ser. No. 08/829,597 entitled "Piezoelectric Resonator and Electric Component Including Same" (Attorney Docket No. 36856.22), the disclosures of which prior applications are incorporated herein by reference, a piezoelectric resonator in which a plurality of piezoelectric layers and a plurality of electrodes forming a base member preferably having a longitudinal shape are alternately laminated, in which the plurality of piezoelectric layers are polarized along the longitudinal direction of the base member, and in which vibration in the longitudinal fundamental mode is generated. This piezoelectric resonator is a stiffened type resonator and is therefore free from the drawbacks in using the above-described unstiffened piezoelectric resonators.
A ladder filter may be formed by using such a stiffened type piezoelectric resonator, for example, as shown in FIG. 21.
FIG. 21 is an exploded perspective view of a ladder filter 4 using a piezoelectric resonator constructed as described in Japanese Patent Application 8-110475 made by the applicant of the present invention, and FIG. 22 is an exploded perspective view of essential elements of the ladder filter 4. In the ladder filter 4 shown in FIG. 21, three pattern electrodes 6a, 6b, and 6c are preferably provided on an insulating substrate 5. Four supporting members 7a, 7b, 7c, and 7d are preferably formed of an electroconductive adhesive on the pattern electrodes 6a to 6c. The supporting member 7a is formed on the pattern electrode 6a, the supporting members 7b and 7d are disposed on the pattern electrode 6b, and the supporting member 7c is disposed on the pattern electrode 6c. Each of four piezoelectric resonators 1a, 1b, 1c, and 1d has a pair of electrodes 3, one of which is mounted on the corresponding one of the supporting members 7a to 7d. The other electrodes 3 on the three piezoelectric resonators 1a, 1b, and 1c are connected to each other by conductor wires 8. The other electrode 3 of the piezoelectric resonator 1d is connected to the pattern electrode 6c by a conductor wire 8. A metallic cap 9 is placed on the insulating substrate 5. This ladder filter 4 has a ladder type circuit shown in FIG. 23.
Because the ladder filter constructed as shown in FIG. 21 requires conductor wires for connection between the resonators and for connection between the resonators and the pattern electrodes, the process of manufacturing the ladder filter is complicated and there is a limit to the reduction in the size of the ladder filter that can be achieved. Therefore, if the pattern electrodes of this ladder filter are arranged close to each other on the insulating substrate to reduce the size of the component, a stray capacitance is created between the pattern electrodes, so that the amount of attenuation achieved by the filter is reduced. The ability to reduce the size of the component is also limited by the necessity to avoid this undesirable effect.